Previous research has identified the motot neuron network that controls the muscle activity giving rise to the traveling body wave of the swimming medicinal leech, Hirudo medicinalis. The swimming rhythm has its origin in the interconnections of a small group of interneurons whos interactions not only generate the rhythm in each segmental ganglion, but also provide for the coordination of the rhythm aaong the entire animal. A mechanism for fhythm generation, recurrent cyclic inhibition, has been shown by theoretical an analysis and modeling studies to account for the period and intra-and interganglionic phase relationships of the rhythmic activity of these interneurons. I propse additional experiments to verify ssme as yet untested predictions of the modeling studies and to complete the identification of the oscillator circuit. The research will then be extended to identify the interneurons that can initiate, terminate or otherwise modify the swimming rhythm. finally, since leech swimming activity can be evoked with small waves in an aquarium, an effort will be made to find the neuron path leading from the sensory transducer that detects the water waves to the oscillatory interneurons generating the swimming movement. The overall purpose in all of these experiments is to use the leech nervous system to develop models describing neural circuits. These models will be applicable to nervous systems generally.